Different rechargeable battery technologies require different charging techniques. A popular battery charging technology is called CCCV or Constant Current Constant Voltage charging. This charging technique uses a controlled current to recharge the battery during the first phase of charging. As the battery nears fully charged the charger voltage reaches a point where it is limited and the current is allowed to fall. This method of charging is most commonly used in ion-exchange systems such as Lithium Ion based batteries.
Systems currently in use fall into two broad categories, hardware based “dumb” chargers which use discrete circuitry to control the CCCV system, or “smart” chargers which communicate with the battery through a digital communication path, or use four-wire voltage sensing at the battery terminals to precisely set the current or voltage outputs from the charge system.
Dumb chargers have the advantage that they are very low cost and are easier to develop. The current is measured and regulated as it exits from the charger system and the voltage is measured at the output of the charger system. The disadvantage of the dumb charger system is that it works best when closely coupled with the battery itself. Therefore operating a dumb charger in a housing that is separated from the battery (such as a charger cradle for a phone) results in a voltage difference between the output terminals of the charger and the battery itself. This can result in longer charge times and reduced charge capacity as the battery charging may terminate early.
Dumb chargers have been improved by implementing a four-wire connection, in this case charge current to the battery is provided on two wires, and the voltage at the battery is measured and returned to the charger on two separate wires. This allows voltage drop in the wires to be compensated for in a very precise way. A four wire system is very accurate and fast at charging the battery and can be used in cases where the charger is separated from the battery by long wires. One disadvantage of this system is the added cost and complexity of the wiring and connectors which now have double the number of conductors. A second disadvantage of this system is poor safety as problems with the returned voltage signal such as broken wires, or current leakage such as from moisture on the connector, can cause the charger system to mistakenly output too high a voltage.
When Lithium based batteries are charged with too much voltage, even just 1% higher than their normal rating, their chemistry can become unstable and in extreme cases the batteries may become hot, vent or even catch fire.
Smart charger systems have the advantage that the battery voltage, charge state and current can be read digitally over a communication bus, therefore resulting in the highest level of safety possible. Charge current can be maximized, thereby reducing charge time. Loss of digital communication can be used as a signal to the charger to disable charging completely which also improves safety. The principle disadvantages of a smart system lie in the cost and complexity. Development time of a smart system tends to be quite long, the system requires multiple micro-controllers (one in the battery and one in the charge system at a minimum) and the number of connections required to the battery is high, often as many as 5 total connections. A further disadvantage of the system lies in the reliance on multiple micro-processors each running embedded software which may have design bugs that could result in an unsafe condition.
There exists a need for a system that provides rapid charging in a CCCV based system without sacrificing safety, and without increasing the number of connections required, especially when used to charge a battery through long wires. There further exists a need for a system that fills the void between a completely “dumb” charging system and a completely “smart” charging system.